1. Field of the Invention
The present invention relates to micro-lenses of image sensors, and more particularly, to defects inspection on micro-lenses of image sensors.
2. Description of the Prior Art
Image sensors are integrated circuit (IC) components that can be used to capture still images or motion pictures. It is well known that an image sensor can be of either the complimentary metal oxide semiconductor (CMOS) type or the charge coupled device (CCD) type. No matter what type an image sensor belongs to, it is made up of a two-dimensional array of light detecting pixels. Up-to-date image sensors usually include millions of pixels in the two-dimensional array so as to provide images with high resolution.
Generally speaking, each light detecting pixel of an image sensor is composed of three main parts: a micro-lens for gathering incident light, a color filter for filtering the light that has already passed through the micro-lens, and a light detecting element set substantially on the focal point of the micro-lens for detecting the light after it has been filtered by the color filter.
Conventionally, micro-lenses are formed by spin coating a layer of micro-lens material onto a surface of the image sensor. The micro-lens material is then developed to form cubical/cylindrical regions or other shaped regions that are centered above each pixel. After the micro-lens material is heated, each cubical/cylindrical region re-flows to form a convex hemispherical micro-lens.
The above-mentioned micro-lens forming process is important and may greatly influence the image capturing ability of the image sensors. For example, in order to increase the light gathering efficiency, each of the micro-lenses should be as large as possible. However, it is undesirable for the individual micro-lenses to contact with each other and thereby interfere. After being heated, if any cubic of the micro-lens material over re-flows, it might contact with another adjacent micro-lens. The unwanted contact between two adjacent micro-lenses caused by over re-flow is referred to as “micro-bridge,” which is a kind of defect that deteriorates the image capturing ability of the image sensor. In short, it is desired that the micro-lenses be as close to each other as possible without the existence of micro-bridges.
In addition, curvature of the micro-lenses is another characteristic that should be carefully controlled. If the micro-lens forming process is performed unsuccessfully, the curvature of the micro-lenses may deviate from a required value, and the image capturing ability of the image sensor will be deteriorated.
In the related art, after a wafer for manufacturing image sensors has gone through the micro-lens forming process, in-line (or in-Fab) product monitoring will be performed on the image sensors to determine whether or not the micro-lenses are properly formed on the image sensors. Ideally, defects such as micro-bridges and incorrect curvatures should be detected through the monitoring process. However, since the micro-lenses are tightly arranged on the image sensors, the monitoring process performed on one micro-lens or a joining point of two micro-lenses will inevitably be influenced by other adjacent micro-lenses. In other words, since the micro-lenses are tightly arranged on the image sensors, it is difficult to determine whether the micro-lens forming process is successfully performed or not through performing the in-line monitoring process.